In recent years, with the evolution of information technologies, the standards and demand for low power and thin display devices have risen, so that extensive investigations have been made on display devices, related equipment and chemical entities fitted to these needs.
Electrophoretic displays known so far generally comprise a multiplicity of electrically charged electrophoretic particles which are dispersed in a space between a pair of substrates, each with one or more electrodes, together with a dispersion medium which is filled in the space and colored in a color different from the color of the electrophoretic particles. In the space between the substrates, a partition wall arrangement is formed so that it divides the space into a multiplicity of pixels along a planar direction alongside and between the substrates. By forming such a partition wall arrangement, it is possible to define the space in between the pair of substrates while preventing complete local fixing of the eletrophoretic particles.
If a voltage is applied between the electrodes, the charged eletrophoretic particles move to the electrode having the opposite polarity to their charge and can, for example, due to the motive power with the force exerted by the electric field, be collected there to cover an observer's side electrode, so that a color identical to the color of the electrophoretic particles is displayed when the electrophoretic display device is observed from the observer's side. Thus, any image (including characters) can be displayed by a multiplicity of pixels.
Today's state of the art concerning e. g. electronic paper, one potential application of the claimed particles, is the already existing black and white or light color electronic paper as a display using electronic inks, bases on bright/dark contrast. Electronic ink is a material that is processed into a film for integration into electronic displays. The principal components of electronic paper devices are outlined in WO 94/28202 and US 2005/0267252. In brief: a multiplicity of charged particles is dispersed in a dielectric medium spaced between two switchable electrodes—one of them being of a transparent material—of opposite charge that can be switched on or off. This array is conveniently divided into a multiplicity of pixels. As already described above in general, in such an electrophoretic display device, when e.g. a negative polarity is applied to a transparent electrode and a positive voltage to the electrode on the opposite side, the positively charged nano-particles will migrate to the transparent electrode and thereby display their coloration to the observer. If the polarity of the electrodes is reversed, the particles migrate to the bottom electrode and the observer will see the coloration of the dielectric medium or the coloration of a second species of particles in the same pixel, however, of opposite charge with respect to the charge of the previous particle species.
It is of importance for electrophoretic displays, especially for electronic paper, that, once some contents are displayed, the display can be retained for a longer period of time even though a voltage is no longer applied. With this approach an image or a text can be visualized practically permanently on displays surfaces.
The main disadvantages of today's available technologies mainly are due to the lack of a truly full color system. Moreover, there is a need for charged colored particles which can be easily dispersed in non-polar media, and which are able to retain their charge upon switches of the electric field. In order to obtain this, colored particles are required which combine several functionalities like charge, steric stabilizing groups, and dyes covalently or irreversibly attached to them.
To replace the black and white state devices by a coloured electronic paper display, it is a requirement to have coloured charged particles (e.g. green, blue and red or magenta, yellow, and cyan) of appropriate size and homodispersity, which can be guided by electrophoretic movements like the black and white particles as described above, when sandwiched, or comprised between a positive and negative electrode.
In order to be applied in any kind of electrophoretic devices, the particles have to comply with a range of requirements. For example for a transmissive type electrophoretic display application, the particle size must be in the nano-meter (nm) range, whereas for a scattering type electrophoretic displays, the particles have to be in the micrometer range. The whole assembly of the particles has to be of a similar to equal size, meaning a highly homodisperse distribution. In addition, the shape of the particles should be of a similar or the same morphology, meaning preferably homomorphous spherical. In order to migrate in an electric field, the particles have to carry a stable, covalently bound, or irreversibly bound defined charge, or chargeable groups. In addition, the particles should be collidal stable, preferably should not settle irreversibly, aggregate or diffuse once the external voltage is turned off, for which it is useful to be capable of adjusting the density of the particles and the dielectric medium in the desired manner. It should also be possible to achieve that the particles possess a brittleness or softness, respectively, to such an extent as to allow to avoid wear off by abrasion when used for an extended period of time and on-off-cycles of the electrical field. Further important demands are good color intensity and color strength of the particles. Furthermore, the particles have to be dispersible in the dielectric medium. Moreover the conductivity of the dispersion comprising the charged particles, has to be minimal, in order to avoid power consumption, and other undesirable effects during the device operation. Yet further, both the electrophoretic dispersion medium and the single chemical components of the particles have to be chemically compatible.
There are several approaches to meet at least some of said requirements, e. g. in WO 94/28202, where organic or inorganic pigment particles are wrapped with a charged polymer. However those particles do not fulfill all requirements. They are only of low homodispersity and are too large, for example, for the transmissive type approach. In addition, the applied radical polymerisation technology for inclusion of the pigment particles and dyes in a charged particle is only rarely compatible with the chemical structure of the dyes and pigments used.
A similar approach is disclosed in US 2005/0267252, where preformed pigment particles are coated with a charged polymer shell. In this case the pigment particles need to be prefunctionalized to be incorporated into the desired particles, which is only possible satisfactorily with the exemplified inorganic pigments. Chemical compatibility with the radical polymerization conditions is rarely given for multi-functional organic dyes and pigments; not to mention the technically difficult targeted derivatisation of the latter compounds in chemical reactions.
There are also numerous reports which describe the incorporation of colored substances—dyes or pigments—into or onto preformed organic particles via a linker on the polymeric particle or the colored component or the copolymerization of functionalized dyes with monomers to form colored polymeric particles (F. M. Winnik et al. Eur. Polym. J. 1987, 23(8), 617-622, or U.S. Pat. No. 6,509,125). In these cases the desired charge of the particles has to be introduced in a separate step; moreover, in many cases the functionalized dye precursor, especially when already charged, is not soluble in the applied liquid monomer components, which leads to phase separation of the colored comonomers and/or incomplete polymerization and formation of large amounts of undesired coagulates (D. Horak et al. J. Poly. Sci.: Part A, Polym. Hem. 1995, 33, 2961-2968).
On the other hand, there a many papers which describe the preparation of charged particles via copolymerizing uncharged and charged monomers in an emulsion polymerisation (Z-Liu et al. Polymer 2000, 41, 7023-7031; W. T. Ford et al. Langmuir 1993, 9, 1698-1703; or F. Ganachaud et al. Polymers for advanced Technologies, vl. 6, pp 480-488, John-Wiley Ltd. 1994). However, as stated in these papers, the ratio of charged versus uncharged monomers significantly influences the rate of polymerization, the yield of polymers and particle size and particle morphology. In any case, there is no general protocol for the synthesis of highly charged particles especially for the nano-meter range. The difficulty is that the mentioned parameters can not be adjusted independently from each other as desirable for the preparation of colored charged particles as stated above for their intended use.
The copolymerisation of charged dyes is possible when less demanding features of products are satisfactory as for instance in bulk polymers (U.S. Pat. No. 6,509,125 or EP 0621 322). In general the synthesis of polymers containing both hydrophobic and hydrophilic—charged—functionalities presents difficulties (U.S. Pat. No. 4,918,123). Thus, the preparation of highly charged small (especially nano-) particles—preferably in the desired low nano-meter range—can not be achieved that way because of electrostatic repulsion which results during the synthesis between the incoming charged dye monomer and the already formed charged particles, which already carry a low charge of the same polarity.
As emulsion polymerisation, which gives well defined homodisperse particles in the nano-meter range, is sensitive to many variables that have to be tuned individually and any change of one variable requires all of the other factors to be adjusted, we chose the following broader approach.
The deficiencies described above provide evidence that there is a strong need for additional electrophoretic functional particles as well as methods for their synthesis and use which avoid or diminish at least some of the disadvantages of the prior art as described above and, in addition, show at least one of the desired features also mentioned above, especially, but not only, stable and good dispersable electrophoretic particles in the nano-meter range.